1) Field of the Invention
The present invention relates to a process for producing biaxially stretched film made of thermoplastics, with a thickness of less than 2 .mu.m, where at least two films are superimposed and stretched together in the longitudinal and transverse directions.
2) Prior Art
Ultra thin films with a thickness less than 3 .mu.m, particularly less than 1 .mu.m are required on ever larger scales for the production of capacitors in microelectronics. In practice, the production of these films presents extraordinary difficulties. In literature, different processes are described aiming mainly to produce first the thin film together with or on a base film, peel off the base film, and then roll up the thin film.
German Auslegeschrift DE-A-17 04 764 (equivalent to Canadian Patent 887,168) describes how two single films can be caused to adhere to one another through pressure and/or heat. Due to unavoidable slight variations in thickness of the single films, creases are formed repeatedly, whereby the creases are further ingrained during the laminating process, which means that such films are useless for their intended purpose.
German Auslegeschrift DE-A-24 31 385 (equivalent to U.S. Pat. Nos. 4,017,227 and 4,076,785) describes a device where two cast sheets are produced next to each other and joined before stretching in the transverse direction. In this method, however, the supporting function of the base layer is missing and breaking of the thin film is frequently observed.
European Patent Application EP-A-0 153 081 (equivalent to U.S. Pat. Nos. 4,708,905 and 4,756,064) relates to stretching coextruded composite films, where the individual layers are made of different polymers, and to delaminating the film composite, after stretching. But with this process, the flow velocity of the different plastics cannot be coordinated exactly. Regulating a uniform thickness profile across the film width also creates great problems since allocating the thickness distribution of the total composite film and the top layer is difficult; moreover, adjustment of the thickness profiles of the individual layers is subject to constant fluctuations at coextrusion.
It is also known that at the time the melt is drawn from the extruder, the molten plastic already has a certain measurable molecular preorientation which is dependent upon technical conditions, like die position, die width, take-off speed, chemical composition of the plastic, and its degree of polymerization. The level of the molecular preorientation and its uniformity, are of considerable importance for the further stretching and for the physical properties of the final film, such as mechanical properties like tear resistance and dimensional stability or the thickness profile and is of particularly crucial importance when producing extremely thin films.